ADHD linked to smog exposure in womb

Researchers report that a child's risk of ADHD symptoms appears to increase dramatically if they were exposed in the womb to high levels of air pollutants.

Pregnant women exposed to air pollution are five times more likely to have children who develop behaviour problems related to attention-deficit/hyperactivity disorder, or ADHD, a new study reports.

More intense symptoms

A child's risk of ADHD symptoms by age 9 appears to increase dramatically if they were exposed in the womb to high levels of air pollutants called polycyclic aromatic hydrocarbons (PAHs), researchers at Columbia University reported.

Compared to children with low PAH exposure, children exposed to high levels are more likely to have both an increased number of symptoms and more intense symptoms, said lead author Frederica Perera, a professor of environmental health sciences at Columbia's Mailman School of Public Health in New York City.

Pollutants and other environmental factors likely interact with an unborn child's genetics to increase their risk of ADHD later in life, said Stephen Faraone, a professor of psychiatry at SUNY Upstate Medical University in Syracuse, New York, who reviewed the study's findings.

"We think it means as the brain is developing, these toxins change something in the functional connectivity or structure of the brain that downstream leads to ADHD," Faraone said. He noted that studies like this prove an association but not a direct cause-and-effect link between air pollution and ADHD.

The study is published in the online edition of the journal PLoS One.

Polycyclic aromatic hydrocarbons are chemicals formed during the incomplete burning of coal, oil, gas, wood, garbage or other organic substances, according to the U.S. Centres for Disease Control and Prevention. Traffic, home heating furnaces and coal-burning power plants are prime sources of PAHs, the researchers noted.

Sustained mental effort

Children exposed to PAHs in the womb were particularly likely to display signs of inattentiveness that are related to ADHD, Perera said.

These symptoms include reluctance to engage in tasks that require sustained mental effort, difficulty maintaining attention in class, an inability to listen to what's being said to them, and a lack of follow-through or completion of tasks, according to the researchers.

The findings from this study build on previous Columbia University studies linking prenatal PAH exposure with behavioural and mental issues, according to a university news release. These include associations with developmental delays at age 3, reduced IQ at age 5, and symptoms of anxiety, depression and attention problems at ages 6 and 7.

The current study included more than 230 non-smoking pregnant women from New York City, and their children.

The researchers tested PAH exposure by looking for the chemicals both in the mothers' blood and blood from the umbilical cords.

"It reflects not only that the chemicals have been brought into the body, they have been activated through metabolism and they have bound to DNA," Perera said.

Five times the odds

The investigators later tested each child's behaviour by having parents fill out questionnaires aimed at detecting ADHD symptoms.

The study authors found that children born to mothers exposed to high levels of PAH during pregnancy had five times the odds of developing many and more severe ADHD symptoms, compared with mothers with little to no exposure.

Pregnant women concerned about PAHs should avoid exposure to second-hand tobacco smoke, stop burning candles or incense in their homes, and use proper ventilation while cooking, Perera said. They also can purchase and use home air-filtering devices, which have been shown to reduce PAHs in the air.

But those sources pale to the exposure that pregnant women receive due to air pollution, she added.

"Air pollution is one of those exposures that's involuntary, and outdoor air pollution doesn't just stay outside the home. It penetrates into the indoor environment," Perera said.

Regulators can play an active role in reducing levels of PAHs in the air, she said. Perera noted that New York City has dramatically reduced its air pollution by getting rid of diesel buses, passing anti-idling laws for diesel buses and trucks, and phasing out the dirtiest home heating oils burned in household furnaces.

"There is some progress in cleaning up the air in the United States, but we have a way to go," she said.

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